Anisotropy of Shielding and Coupling in Liquid Crystalline Solutions

Abstract

NMR spectroscopy of molecules dissolved in liquid crystals (LCNMR) provides a method to investigate molecular structures, nuclear shielding tensors, indirect spin-spin coupling tensors, as well as quadrupole coupling tensors. This is because in liquid-crystalline solutions, intramolecular dipole-dipole couplings and the above-mentioned tensors do not average to their isotropic values, which is because of the anisotropic orientational distribution of solute molecules. Therefore, more information is available about the tensorial properties than is available in isotropic environments. An advantage of carrying out experiments in liquid crystals (LCs) is that the resulting NMR spectra resemble those obtained in isotropic solutions; spectral linewidths are typically a few hertz or, in most favorable cases, even below 1 Hz. A restriction in turn is that only small- or medium-sized molecules can be studied in strongly orienting thermotropic LCs. On the other hand, in weakly orienting media even NMR investigations of biomacromolecules have become feasible. This article focuses on the description of various LCNMR methods to gain information on nuclear shielding and spin-spin coupling tensors. The derivation of reliable and accurate results requires consideration of vibrational effects as well as deformational effect due to the correlation between intramolecular vibrational motions and reorientational motion. Keywords: liquid crystal; dipole-dipole coupling; residual dipolar coupling; shielding anisotropy; chemical shift anisotropy; residual chemical shift anisotropy; spin-spin coupling anisotropy; vibrational effects; deformational effects